The invention relates to an image forming apparatus such as copier, printer, facsimile, scanner, and the like that can reads image information on documents. More particularly, the invention relates to an image forming apparatus equipped with an automatic document feeder that can take up each document from a document bundle placed on a document tray and reads image information of the document by a reading means.
Recently various information-processing technologies have made a rapid advance. Such technologies are a technology for photo-electrically reading image information (such as characters, numerals, symbols or pictures, drawings, and photos) on documents, a technology for storing a lot of read information, converting it into, for example, digital data, and storing it in a storing means such as memory, and a technology for transferring and reproducing digital data by mans of a transmission. These technologies have enabled image forming apparatus such as copier, printer, facsimile, scanner, and the like to process a lot of document image information very fast. Consequently, such image forming apparatus have been requested to handle much more documents.
Generally, an automatic document feeder (ADF) is often used to read a lot of documents.
The automatic document feeder has a document tray, a document conveyance device containing a plurality of rollers, and a control circuit that controls the operation of the document conveyance device to pick up each document from a document bundle placed on the document tray, automatically convey the document to a reading means of the image forming apparatus, and enable the reading means to easily read image information on one or both sides of the document.
In the automatic document feeder, a single document is taken up from the document bundle placed on the document tray by a document feeding means such as a take-up roller and transferred to the reading position of the reading means in the image forming apparatus through a long document path by a document conveyance device composed of a pair of rollers such as registration rollers and feed rollers. In this document conveyance, the document may be inclined from the direction of the normal document conveyance. This phenomenon is called a “document skew.”
Such a large document skew may cause a paper jam in the document path of the automatic document feeder or may break the document due to document fold.
Conventionally, the automatic document feeder corrects a document skew by detecting the document path by sensors provided along the document transfer path, for example, by measuring a document conveyance time, controlling the operation of the document conveyance having a plurality of motors and rollers, and hitting the leading edge of the document to the peripheral surfaces of the rollers which have been stopped. (For example, see Patent Document 1: Japanese Non-examined Patent Publication No. Hei 08-272161.)
However, the large amount of documents in a bundle may not always be of the same size. They may contain paper sheets of various sizes. The recent automatic document feeders have been demanded to handle document bundles containing a lot of paper sheets of various sizes.
If the above method disclosed by Patent Document 1 is used to convey documents without skews, the automatic document feeder must detect the conveyance status of documents of each size. For this purpose, the automatic document feeder must have a great many sensors to detect various document sizes and the control circuit must be complicated to finely control the operation of the document conveyance device having a plurality of motors and rollers by detection signals from such a great many sensors.
Such a complicated control makes the document conveyance device more complicated equipped with additional sensors and motors. This increases the production cost of the automatic document feeder and makes it expensive. Naturally, the image forming apparatus with an automatic document feeder cannot be inexpensive.